Synthesis of CNS, ZnO/CNS and ZnCr2O4/CNS composites from patchouli biomass by using microwave for remediation of pesticide contaminated surface water in paddy field

Patchouli biomass is a potential precursor for CNS synthesis. In this research, the patchouli was pyrolyzed using the microwave. The purpose of this research is to study the effect of microwave energy and activator toward physicochemistry of CNS and composite (ZnO/CNS) and application of ZnCr2O4/CNS for the pesticide polluted surface water remediation in paddy field. In the process, the biomass was pyrolyzed at four and 8W with and without the ZnCl2 activator. The products were blended and evaporated to obtain CNS and ZnO/CNS. The products were characterized using FTIR spectrometry, XRD, and dispersion test. The composites were used to synthesize ZnCr2O4/CNS at 600W in the microwave. The composites were used for buthylphenylmethyl carbamate pesticide degradation test (BPMC) for 48 h with H2O2 oxidation. The FTIR spectra indicated better carbonization for products taken using an activator at both microwave energies. The X-ray diffractograms showed the turbostratic structure of carbon obtained at 4W pyrolysis (with activator), meanwhile 8W pyrolysis (without activator). ZnO and turbostratic carbon structures were shown by the product of 8W pyrolisis with activator. The calcined composite indicated ZnCr2O4/CNS. The degradation test showed that ZnCr2O4/CNS(8W) catalyst decreased the BMPC concentration almost three times that of the composite (4W).

Among them, organochlorine and organophosphate are very toxic and have long persistence. Carbamate and piretroide are easier for degradation but keep potentially accumulated [2]. Accumulation of pesticides lasts in both water and soil. It can be toxic and kills animals, but on the other side can create pest resistance. The other risks of pesticides include birth disability, genetic mutation, brain damage, and cancer. Pesticide residue potentially contaminates various agricultural products [2,3]. The pesticide pollutant problem can be overcome by using nanocarbon adsorbents. Nanocarbon or carbon nanomaterial has a particle size of 1-100 nm [4]. The small size of this carbon particle makes it easier for farmers to spray it on paddy fields together with liquid fertilizer.
Nano carbons have been prepared using the pyrolysis method of hydrothermal [5][6][7][8], conventional thermal [6,[9][10], and microwave thermal [9,11]. Hydrothermal carbonization (HTC) is a thermochemical process that lasts in a sub-critical liquid water medium at a temperature of 180-250 o C. This method applies lower operating temperature than the pyrolysis process by the conventional furnace [12]. Hydrothermal pyrolysis usually uses acids, bases, metallic salts, or water substances as green catalysts. To transform the biomass into carbon, a chemical reaction in hydrothermal pyrolysis is complex, such as condensation, hydrolysis, dehydration, and decarboxylation. The hydrothermal carbonization (HTC) process is conducted in the autoclave. A vessel made from teflon inside and stainless steel outside. By this autoclave, pressure can be created autogenously. Hydrothermal pyrolysis is more flexible than dry pyrolysis. The wet biomass can be used to exclude the pre-drying process before the HTC treatment [13].
A microwave (MW) is electromagnetic radiation that has a frequency of 300-300000 MHz. Electromagnetic radiation passes through the material and stimulates molecule oscillation which produces heat. Microwave heating produces heat in the entire material at a high rate of reaction. It leads to a fast reaction [14]. The property of microwave heating is different from conventional heating. Conventional heating involves the usage of a furnace that heats the walls of the reactors by conduction. It needs a longer time to heat material than the microwave because the microwave penetrates inside the material, and the heat is generated by direct interaction of microwave and material [15]. Consequently, a microwave has a short duration of pyrolysis [5,6,7,9,10].
Nanocarbon can be produced from biomass. The biomass is a lignocellulosic material rich in cellulose, lignin, and hemicellulose, i.e., 20-40, 40-60, and 10-25 wt.%, respectively [16]. Biomass can be transformed to carbon using thermal, biological, and physicochemical processes [13]. Thermally, stages of biomass pyrolysis involve moisture evolution and decomposition of three lignocellulose components [16]. Some biomasses have been used as precursors of the nanocarbon using the hydrothermal -microwave method, such as patchouli, rice husk, and sugarcane leaf [17]. The other research has used those same biomasses and two more others, including sawdust and coconut husk, by microwave pyrolysis [18]. Oxide metal nanoparticle was also synthesized using microwave calcination, such as ZnO [19].
Nanoparticles form the typical colloidal systems, consisting of solvent as the dispersing phase and nanoparticle as the dispersed phase [20]. The colloidal suspension of nanoparticles that are less than 100 nm are dispersed in base fluids such as water is called nanofluid [21]. Some techniques can be used to conditionate nano size of carbon nanomaterial to form dispersion system, i.e, a technique of sonication [17,22,23] and blending [17]. Based on the stability test of carbon colloid, the blending technique resulted in better stability of carbon colloid than sonication [18].
Degradation of pesticides in the soil can be conducted naturally microorganisms such as bacteria, fungi, and actinomycetes are the essential group of degraders. The pesticide degradation by biological oxidations is accomplished enzymatically, primarily by the mixed function oxidases (utilizing cytochrome P450), the flavin-dependent monooxygenase, and monoamine oxidase [24]. Chemical degradation can be performed friendly using oxidator such as peroxide acid. This oxidator is an additive in the liquid fertilizer for removing fungi or bacteria which inhibit plants growth [25]. Peroxide acid is safe for fish and can improve DO in a water environment [26].
The composites of MFe2O4/CNM and MCr2O4/CNM (M = Zn, Ni, Mn) have been used for catalytic degradation of BPMC pesticide. CNM was synthesized with hydrothermal -microwave methods using biomass such as sugarcane leaf, patchouli, rice husk, and wood char. Both ZnCr2O4/CNS and ZnFe2O4/CNS gave good catalytic activity in dark degradation [17]. In this research, the patchouli biomass was pyrolyzed using a microwave with a ZnCl2 activator. The product was used to synthesize ZnCr2O4/CNS. The final product was applied as the catalyst for the degradation of pesticides.

Procedures of research 2.2.1. Synthesis of ZnO/CNS and ZnCr2O4/CNS
The dry patchouli biomass was shieved by tea shiever then mixed with ZnCl2 at a ratio of 5:1 and pyrolyzed at 400, 600, and 800W for 50 minutes. The products were blended in water to form colloids. The colloids were used for the test of stability and evaporated to obtain ZnO/CNS nanocomposite. For comparison, the CNS was prepared in the same procedure without ZnCl2. Each ZnO/CNS composite (1.0 g) was further calcined at 400 and 800 W after mixing with some substances including KOH (1.34 g), ZnCl2 (0.20 g), CrCl3.6H2O (0.79 g), and distillated water (1 mL), then calcined by microwave for 5 minutes at 600 W to produce ZnCr2O4/CNS composite.

Characterization
The ZnO/CNS products were characterized with some methods such as FTIR spectrophotometry (functional group), X-ray diffraction (crystal structure), SEM (morphology), and optic photography, and TDS meter for the test of colloid stability. The final product, ZnCr2O4/CNS was characterized with FTIR spectrometry.

Application of ZnCr2O4/CNS composite for dark degradation reaction of pesticide
Before application, paddy soil was deactivated by drying at 200 o C for five h and conditioned to particle size > 200 mesh and homogenized. The pesticide (BPMC) solution was prepared by dissolving the concentrated BPMC (500 g/L; 0.5 mL) to 500 mL. The oxidator H2O2 solution (0.15%) was prepared by dissolving more concentrated H2O2 (3%; 25 mL) to 500 mL. Test of dark pesticide degradation in the soil was conducted by procedures presented in Table 1 as follows:

Material transformation
Patchouli biomass was used as a precursor for the synthesis of ZnO/CNS. Besides that, the biomass was analyzed to determine its content. The analysis laboratory determined lignocellulosic components in the patchouli biomass for public service in the Department of Chemistry Brawijaya University. The biomass photograph and result of the analysis are presented in Figure 1. Based on the analysis, it is known that lignocellulose content in the patchouli biomass is in a sequence of cellulose > lignin > hemicellulose. Those three substances are different macro molecules with different structures, but all are organic polymers rich in hydroxide functional groups. Among them, only lignin contains the aromatic structures which are linked by oxygen or carbon atoms, whereas the two others consist of alkane cyclists that are linked by oxygen atoms only [27].

Figure 1. Patchouli biomass and component in patchouli biomass
The pyrolysis process of the biomass using a microwave for 50 minutes resulted in the black products named activated carbon ( Figure 2). This carbon can be called activated carbon because ZnCl2 was used as the activator. The role of this salt is a dehydrating agent, which improves reactions in pyrolysis and reduces the side product such as tar [28]. No washing was conducted so that the pyrolysis product contains ZnO due to the reaction between ZnCl2 and gasses emitted by the reaction. Based on color of the carbons, the carbon which was pyrolyzed without the activator, especially at the lowest energy, i.e., 400 W (NM4), has the lightest color (brownies black). The carbonization reaction is worst because the process was conducted at the lowest energy or the lowest temperature. The Efforts to optimize the nano size, the carbons were blended in water to form the colloid or dispersion systems. The mechanical force of the blending technique caused the collision of the carbon particles. The particles broke and formed the smaller ones. The carbon colloids which were resulted from blending are presented in Figure 3.  The TDS data shows that pyrolysis with the ZnCl2 activator causes TDS of the carbons (NMZ4, NMZ6, NMZ8) higher than without an activator. It means that usage of activator causes a higher concentration of the carbon colloid than without the activator. The role of the activator, which prevents the side products (tar) caused an amount of the activated carbon is more than without activator so that concentration of the colloid and TDS are large.
To get the nano carbon with ZnO impurity or ZnO/CNS, the colloids were evaporated. The composites as a result of the evaporation process are shown in Figure 5. All evaporated carbons show smother powder than before dispersion. It is visual proof that the blending process reduced the carbon particles. The color of the composites is matched the activated carbon before the dispersion process.
The ZnO/CNS was used to synthesize ZnCr2O4/CNS by calcination of ZnO/CNS, KOH, ZnCl2, CrCl3.6H2O mixture. During calcination process, there are 2 kinds of reactions, i.e reaction of the substances thermally to form spinel (ferrite) ZnCr2O4. The possible stoichiometric reaction as follows: ZnO + 2CrCl3.6H2O + KOH + ZnCl2 ZnCr2O4 + 7HCl (g)+ KCl + ZnO + 9H2O (g) Another reaction is the further activation of carbon by inorganic substances thermally. The products after calcination are shown in Figure 6.  lowest (400 W) energies were considered to influence microwave energy toward the functional groups significantly. The FTIR spectra of the composites are presented in Figure 7.
Generally, the carbons which were prepared without an activator show sharper FTIR spectra than with an activator. The existence of those sharper bands indicates that the carbonization reaction without an activator is worse than with the activator. So that the functional groups on the edges of the graphene layers are still many, and the spectra bands are sharper. However, the different microwave energy does not have too significant an influence on FTIR spectra.
The sharp FTIR spectra of the carbon were prepared without an activator. The bands related to oxy functional groups can be interpreted as follows: the -OH functional groups of phenol or hydrate at about 3500 cm -1 , C=O at 1700 cm -1 , and C-O at about 1000 cm -1 . Those bands match with vibrations of the functional groups in structures of lignin, cellulose, and hemicellulose, such as O-H stretching of phenol at 3000 -3600 cm -1 , C=O stretching at 1700-1730 cm -1 , C-O stretching at 1215 cm -1 (phenol), C-O stretching of pyranose ring skeletal at 1170 and 1082 cm -1 [16]. The existence of those oxy functional groups indicates that the carbonization reaction is not completed.
Nonoxy functional groups detected in FTIR spectra of the carbon are as aliphatic C-H at about 2900 cm -1 , aromatic C=C at 1600 cm -1 , and C-H out of a plane in graphene aromatic layer at about 800 cm -1 . All of those were also detected in the analysis of lignin, cellulose, and hemicellulose, including C-H stretching (alkyl, aliphatic) at 2860-2970cm -1, C=C stretching of benzene ring 1613 at 1632, and C=C stretching for aromatic skeletal mode at 1450 (w) and C-H stretching of C-H aromatic hydrogen at 700-900 (m) [29].

Crystal structure of the carbon and composite
Identification of functional groups is not enough for the Characterization of carbon material, especially related to the crystal structure of a material. This property can be detected by XRD method. The Characterization was performed in LSUM too. The samples were coded by author and by LSUM (in the bracket). Patterns of the carbons and composites' X-ray diffractograms are presented in Figure 8.
The patchouli biomass X-ray diffractogram pattern in Figure 7 is similar to the X-ray diffractogram of biomass and cellulose [30][31]. After pyrolysis at 800 W without an activator (TNM8), the X-ray diffractogram reduces the main peak, which indicates that biomass has been  [32]. Usage of activator at the same temperature shows a different pattern, i.e., the emergence of sharp peaks indicating oxide metal (ZnO) from the reaction of activator and oxy gas released from carbonization reaction. This emergence lasted together with much reduction of the carbon's wide peak. When the temperature was decreased at 400 W with the activator the main peak kept high, and relatively no peaks of ZnO. Both phenomena indicate degradation of carbon in 800 W and a reaction of metal oxide formation.

Colloid stability of ZnO/CNS stability
The stability of the carbon colloid is an essential factor in the application of the composite as an additive of liquid fertilizer. It was performed in this research by observation the colloid from 10.11 AM to 12.28 PM. Changing of the colloid appearance was documented by photography as presented in Figure 10. Figure 10 shows that the colloids of carbons from pyrolysis without activator show more concentration than activator at 11.15 AM and 12.28 PM or after staying 1 and 2 h. It means that the carbons which were prepared without an activator can form more stable colloids. Different concentrations of the colloids cause this different stability after the blending process. Based on Figure  5, the colloids of the carbons by pyrolysis with activator had more concentrated than without activator. Colloidal agglomeration is easy to occur if the colloid concentration is large, the space between the carbon particles is small, and strong attraction between the particles. Based on the FTIR spectra, the surface of the carbons can interact with each other by hydrogen bond and London force.

Functional group of ZnCr2O4/CNS
The final product in this research was characterized with FTIR spectrometry to identify their functional groups. Their functional groups were identified using FTIR spectrophotometry ( Figure 11). The measurement of the FTIR spectra in Instrument Laboratory in Department of Chemistry Brawijaya University. No additional codes of samples from the laboratory. Based on XRD characterization of the final product taken with the same procedure but different biomass (rice husk), the final product was ZnCr2O4/CNS [18]. The different biomass gave the same crystal structure and crystallinity for synthesizing MnCr2O4/CNS, using hydrothermal -microwave method [17]. So that, the final product in this research was predicted as ZnCr2O4/CNS. The functional groups of the final composite were based on reaction equation (1). The calcination gives possible functional groups of ZnO and ZnCr2O4 such as M-O and CNS such as -OH, C=C, C-O, C-H out of a plane. These functional groups of the CNS are products of further activation by salts and bases during the calcination process. Those base and salts such as KOH and ZnCl2 further activated carbon [33][34][35], or CrCl3 oxidized the carbon [36].
To prepare composite of ZnCr2O4/CNS, the mixture of base (KOH), salts (CrCl3.6H2O, ZnCl2), and intermediate composite (ZnO/CNS) have been calcined to get the final composite of ZnCr2O4/CNS. The CNS is eroded by reaction with KOH and leaves voids on its frame. This reaction involves a dehydration reaction of KOH thermally and releases water vapor. This H2O molecule reacts with the carbon surface to form hydrogen gas. The activation reaction of carbon surface by KOH is as follows [29]: 4KOH 2K2O + 2H2O C + H2O CO + H2 CO + H2O CO2 + H2 The ZnCl2 activator is a template that forms pores and an acidic catalyst that supports carbonization reaction [9]. In chemical activation, carbonization and activation work simultaneously [28]. A low melting point (283−293 °C) allows ZnCl2 has better contact with the carbon surface [33]. Chemical activator dehydrates precursor for carbonization and aromatization to form pores and surface area [37]. The dehydration influences pyrolytic decomposition, which impedes tar formation and improves carbon products [38].
Interpretation of spectra functional groups in FTIR spectra of the final products is listed in Table 2. Based on Figure 11 and interpretation in Table 2, the FTIR spectra of the ZnCr2O4/CNS composite from the carbon by pyrolysis at 400 W (CTNMZ4) tends to show sharper bands in vibrations in carbon structure than at 800 W (CTNMZ8), especially related to -OH, C=O, and C-O. 800 W gave higher temperature, which potentially increased the activation process by some substances of base and salts.

Test of pesticide degradation
To support of characterization data for synthesized material, a degradation test of pesticide was conducted. This test result can describe how the performance of the composite as a catalyst in pesticide degradation. BPMC pesticide was used, and the wet paddy soil was chosen as media for an experiment. Filtrates and residues of wet soil after the degradation test are shown in Figure 12. TOC was the measurement for each sample using a mini TOC meter ( Figure 13). 12 TOC infiltrate after the dissolution was measurement using a mini TOC meter and presented in Figure 14 as TOC and percentage TOC reduction. The TOC reduction percentage describes the activity of catalytic oxidation.  Figure 14 shows three different conditions, i.e., the pesticide was oxidized without composite catalyst (BT), with composite which was prepared by involving pyrolysis at 800 W (CTNMZ8) and 400 W (CTNMZ4). Based on the graph, the CTNMZ8 gave a % TOC reduction more than two times. It means that CTNMZ8 has catalytic activity larger than CTNMZ4. The degradation reaction of pesticide is assumed same as DNA damage reaction catalytically by H2O2 using Cr(III) catalyst. The Cr(III) cations reduce H2O2 to form OH . Radicals and the Cr(III) are oxidized to form reactive Cr(IV) [41]. The radicals may attack the pesticide molecules to form more specific degradation products [42].

Conclusions
Synthesis of CNS and the composites including ZnO/CNS and ZnCr2O4/CNS was conducted. The chemical activator of ZnCl2 made darker carbon products, more concentrated colloids, less stable colloids, less functional groups of carbon surfaces, and higher crystallinity of ZnO in the intermediate product. Increasing energy gave no significant different functional groups of ZnO/CNS but decreased functional groups of ZnCr2O4/CNS. The higher the microwave energy, the lower TDS of the carbon colloids, the larger catalytic activity of ZnCr2O4/CNS in dark degradation of pesticide.